QAM (Quadrature Amplitude Modulation) is a digital encoding scheme used to transmit data over Radio Frequency (RF). For example, QAM is used for Digital TV (DTV) video transmission (both live and VOD) over last mile (e.g. Fiber/Coaxial) from a CMTS (which has a QAM subsystem sending out data onto the RF plant) and a Cable Modem (which has a QAM subsystem to receive and demodulate the data) at customer premises.
It is typically referred as a way to distribute video in a “traditional” way in opposition with the news ways to distribute video directly over IP.